With use fluid filters become clogged and lose their effectiveness. Periodic filter inspection to determine the status of the filter is required in most filter applications. Typically, the filter is removed from a fluid flow path and physically inspected. If dirt or other materials have accumulated to clog the filter, the filter is replaced.
Certain applications make the routine inspection of the filter condition unsatisfactory due to the complexity in performing the inspection. If due to the construction and use of the filter, routine inspection becomes more difficult, other techniques for determining the filter status are required.
Techniques are known in the prior art for determining the condition of a filter without actually removing the filter from its fluid flow path. U.S. Pat. No. 4,272,368 to Foord et al. relates to a technique for monitoring the condition of a filter. The technique involves the monitoring of a pressure difference across the filter. In the event a specified pressure differential is sensed this pressure differential is assumed to be caused by a clogged filter. Techniques such as the one disclosed by Foord et al. require pressure sensors to be installed on opposite sides of the filter. This can be expensive and may not be justified depending upon the filter application.
U.S. Pat. No. 4,708,790 to Bray discloses a different filter monitoring system. This system monitors fluid flow rates and associates a clogged filter with a reduced flow rate. More specifically, when the flow rate through the filter drops below a specified threshold level, a fluid flow path is adjusted so that the filter can be cleaned and fluid again routed through the regenerated filter.
Such a flow rate sensing system is appropriate so long as fluid flows through the filter at a constant rate. For a system where variable flow rates are anticipated, however, such a system is ineffectual. In addition, the use of sophisticated flow meters for monitoring a filter condition is too expensive for many applications.